Loading of fluid to tankers in open sea may be a demanding operation, in particular in harsh environment. The operation requires dedicated shuttle tankers equipped with dynamic positioning system, excessive thruster capacity and specialized loading systems. Such shuttle tankers are equipped with loading systems, normally installed in the vessel's bow, enabling the tanker to connect to a floating production facility, a loading tower or loading buoy via a loading hose, and thereby allowing transfer of the cargo to the tanker. The tanker can be moored to the production by a flexible hawser, assisted by vessel's own thrusters or propellers. The tanker can alternatively be positioned by its own thruster system (Dynamic Positioning System) without any mooring hawser.
The most advanced system for loading tankers is the proven Submerged Turret Loading, STL, where the tankers is connected to the transfer line of cargo through the vessel's bottom by a rotating buoy moored to sea bed, as e.g. disclosed in WO 95/08469. The STL system allow operation all year round in the most exposed and harsh environment such as the North Sea and North Atlantic regions. Typically for these systems are dedicated ships with additional special designed equipment, resulting in higher investment compared with conventional tankers.
In more benign areas, offshore loading with conventional tankers can be performed using moored floating buoys (Catenary Anchor Leg Moorings, CALM Buoys) moored to the seabed. See e.g. WO 2012/035354. Loading of tankers with CALM buoys are limited by the sea state, current and wind.
The main challenge using conventional tankers are their limited maneuvering and station keeping capabilities. Lately the Hiload concept was introduced to the market. See e.g. WO 2005/118389 A1. The Hiload is a self-contained semi submerged construction with propellers and thrusters. The unit is capable of attaching to the tanker's hull, thereby assisting the tanker's maneuverability. The Hiload requires a dedicated support vessel to assist the Hiload in idle periods and a specialized crew when in operation.
A system that addresses the above disadvantages is disclosed in U.S. Pat. No. 5,803,779. A loading buoy in the form of a floating hull is provided with hawser lines, propulsion means and liquid transfer means to ensure safe liquid transfer operations at a predetermined distance from the offshore structure. However, the disclosed system is considered vulnerable to environmental induced movements such as roll, in particular during liquid transfer. In addition, the suitability for use as an effective means of transport is questionable.
There is therefore a need to mitigate the disadvantages with the existing systems and to further reduce the investments in extra equipment.
It is thus an object of the present invention to provide a method and a system that further improves the loading efficiency of conventional tankers, LNG carriers or other ships carrying fluids in open sea.